Turbine shut-down procedures are important and regularly performed processes that have to be followed through safely and in a controlled and reliable manner. During the run-down of the engine, the turbine is not working productively; the resultant dead time shall be kept, for technical and economic reasons, on a minimum.
US 2010/0275608 A1 provides for a method for shutting down a gas turbine engine system, the method including the steps of reducing a flow of fuel to a combustor, reversing the operation of a generator so as to apply torque to the rotor, and increasing the deceleration of the rotor so as to limit a flow of air into the gas turbine engine system. For that specific purpose, a starting system, which is generally provided in turbine systems for the run-up of rotary parts to firing speed, is intended to reverse the operation of the generator so as to apply negative torque to the rotor while the fuel flow is reduced during shut-down procedures.
Eventually, however, after gradually decreasing the fuel flow over time, the fuel flow may be interrupted completely (flame off) and, at some point in the shut-down schedule, the generator may be disconnected, from the grid (generator breaker and/or high-voltage breaker may be opened). Thereupon, the engine is spinning freely and slowly decelerates through bearing friction, aerodynamic forces on the turbine blading and rotor as well as through ventilation and electric losses in the generator. Importantly, after flame off (i.e. after interruption of fuel flow), there is still a large amount of kinetic (i.e. rotational) energy (e.g. up to several megawatt-hour in the case of large gas turbines) stored in the turbine engine. Moreover, after flame off, the run-down process of a large industrial gas turbine may last approximately another 30 minutes until a target speed, e.g. the turning gear speed, or a stand-still of the turbine rotor is finally reached. The time required for this last step, i.e. the reduction in kinetic energy of the freely spinning engine after flame off, adds significantly to the overall dead time of the turbine engine.
There are several negative issues or problems associated with the run-down procedure of a turbine engine, a few of which shall be mentioned in the following:                i. The turbine is not available for immediate restart during the run-down process.        ii. During the run-down, airfoils or other parts of the turbine or the generator will pass through their resonance frequencies and will accumulate load cycles with possible damages after a high number of shut downs.        iii. The turbine will cool down through the airflow in the engine during the run-down time. Consequently, with a subsequent (immediate) restart after the rotor has a rotation speed equal to or smaller than the respective firing speed, substantial thermo-mechanical stresses on the turbine engine occur.        iv. During the run-down the casings and vane carriers cool down faster as compared to the turbine rotor (the latter having, in comparison, larger material thickness and hence cools down at a lower rate). This leads to stronger shrinkage of the vane carriers as compared to the rotor and a reduction of clearances between rotating and static parts. In some cases blocking of the rotor due to “negative” clearances (i.e. too small clearances) is observed. Therefore, part clearances have generally to be set larger than desired in view of minimum cold-build clearances so as to accommodate these thermal transients; naturally this goes at the expense of the overall performance of the engine.        v. During black run-down (i.e. no in-feed from the grid is available) the engine consumes auxiliary power typically supplied by batteries (or generally by battery elements). These battery elements have a considerable cost impact on the turbine engine.        